1. Field of the Invention
Embodiments described herein relate generally to magnetic resonance imaging.
2. Description of the Related Art
MRI is an imaging method which magnetically excites nuclear spin of an object (a patient) set in a static magnetic field with an RF pulse having the Larmor frequency and reconstructs an image based on MR signals generated due to the excitation. The aforementioned MRI means magnetic resonance imaging, the RF pulse means a radio frequency pulse, and the MR signal means a nuclear magnetic resonance signal.
A gradient magnetic field generation system in an MRI apparatus includes a gradient magnetic field coil which adds spatial positional information to MR signals by applying a gradient magnetic field in an imaging space and a gradient magnetic field power supply which supplies electric power to the gradient magnetic field coil (see, for example, Japanese Patent Application Laid-open (KOKAI) Publication No. 2010-75753).
The gradient magnetic field power supply includes, for example, an electrolytic capacitor which is charged before performance of an imaging sequence and supplies electric power at the time of performance of the imaging sequence.
Capacitance of an electrolytic capacitor governs output specifications of a gradient magnetic field power supply, and influences conditions of an imaging sequence. In general, an electrolytic capacitor used for power source circuits supplying a large amount of power dries up and degrades as long as it is used. The above “dry up” means that desiccation of a separator progresses, because electrolysis solution enclosed inside the electrolytic capacitor evaporates due to heat generation and this makes some parts insufficiently impregnated with the electrolysis solution. If the “dry up” progresses, fibers constituting the separator are peeled off and the capacitance of the electrolytic capacitor becomes smaller.
Therefore, if circuits of a gradient magnetic field power supply and condition setting programs for imaging sequences are designed based on the capacitance value of the electrolytic capacitor of initial installation time, problem-free MRI images can be obtained around the time of the initial installation, whereas there is a possibility that image quality degrades ten years later.
Thus, in conventional technology, circuits of a gradient magnetic field power supply are designed to have margin of, for example, −20% to a nominal capacitance value considering time degradation.
If circuits of a gradient magnetic field power supply are designed so as to have a margin, manufacturing cost of each component increases. Additionally, if the capacitance of an electrolytic capacitor has a sufficient margin by growing the electrolytic capacitor in size and in capacitance based on the theory that capacitance of a capacitor is proportional to surface area of dielectric substance, a gradient magnetic field power supply unit grows in size and its installation space in an MRI apparatus expands.
On the other hand, imaging of high image quality is desired and load on the gradient magnetic field power supply becomes larger in imaging of high image quality.
Thus, in MRI, technology to enable designing circuits of a gradient magnetic field power supply without taking too much margin for capacitance degradation of a charge/discharge element for supplying power such as a capacitor in the gradient magnetic field power supply has been desired.